The present invention relates to polarizer reflectors and to reflecting plate type scanning antennas including such polarizer reflectors. The invention is particularly applicable to the type of scanning antenna, sometimes called the Elliott Cassegrain Scanning Antenna, in which the movement of the antenna beams is controlled by movement of a flat reflecting plate, and is therefore described below with respect to such an antenna.
This type of scanning antenna has been known for about 30 years. Briefly, it includes a feeder for feeding plane polarized electromagnetic waves, a collimating paraboloid disposed in front of the feeder means for forming a collimated plane polarized beam, and a flat reflecting plate disposed behind the collimating paraboloid for producing a reflected beam polarized at right angles to the incident beam from the collimating paraboloid. Thus, the collimating paraboloid forms a collimated plane polarized beam as in a normal horn-and-dish type antenna; while the flat reflecting plate reflects the collimated beam according to the laws of geometrical optics (i.e., the angle of incidence is equal to the angle of reflection), but at the same time, it "twists" the plane of polarization through a right angle. Scanning is achieved by moving the reflecting plate. This provides one of the main advantages of such an antenna since it obviates the need for moving the collimating paraboloid or the feeder. Such an antenna is particularly advantageous where multibeam operation is required, e.g., in a monopulse system, as it obviates the need for rotary joints.
In a known construction of the reflecting plate type scanning antenna, the reflecting plate, sometimes called a "twist reflector," usually employs an array of parallel wires or strips whose front surface is approximately a quarter wave length from a conducting metal back plate. Such an antenna operates on the principle that the incident electric field, polarized at 45.degree. to the wires or strips, is resolved into two waves of equal magnitude, polarized parallel and perpendicular, respectively, to the wires or strips. Most of the energy polarized parallel to these wires or strips is reflected back by them, and the energy polarized perpendicular to the wires or strips is transmitted to the back plate where it is reflected. The phase delay of the latter wave is arranged to be 180.degree. relative to the former, so that, when it recombines with the waves reflected by the wires or strips, the resultant wave is polarized at a right angle to the incident wave.
One of the main drawbacks of the known reflecting plate type scanning antennas is that it is operable over a relatively narrow frequency band. Thus, the known constructions usually operate over a ten percent frequency band, this being mainly attributable to the construction and operation of the reflecting plate or twist reflector disposed behind the collimating paraboloid.